Electroluminescent devices (hereinafter also referred to as EL devices) contain spaced electrodes separated by an electroluminescent medium that emits electromagnetic radiation, typically light, in response to the application of an electrical potential difference across the electrodes. The electroluminescent medium must not only be capable of luminescing, but must also be capable of fabrication in a continuous form (i.e., must be pin hole free) and must be sufficiently stable to facilitate fabrication and to support device operation.
Initially organic EL devices were fabricated using single crystals of organic materials, as illustrated by Mehl et al U.S. Pat. No. 3,530,325 and Williams U.S. Pat. No. 3,621,321. Single organic crystal EL devices were relatively difficult to fabricate and further did not readily lend themselves to thin film constructions.
In recent years preferred organic EL devices have been constructed employing thin film deposition techniques. Using an anode as a device support, the organic electroluminescent medium has been deposited as one or a combination of thin films followed by the deposition of a cathode, also formed as a thin film deposition. Thus, starting with the anode structure, it is possible to form the entire active structure of an organic EL device by thin film deposition techniques. As employed herein the term "thin film" refers to layer thicknesses of less than 10 .mu.m, with layer thicknesses of less than about 5 .mu.m being typical. Examples of organic EL devices containing organic electroluminescent medium and cathode constructions formed by thin film deposition techniques are provided by Tang U.S. Pat. No. 4,356,429, VanSlyke et al U.S. Pat. Nos. 4,539,507 and 4,720,432, and Tang et al U.S. Pat. Nos. 4,769,292 and 4,885,211.
While the art has encountered little difficulty in constructing fully acceptable stable anodes for internal junction organic EL devices, cathode construction has been a matter of extended investigation. In selecting a cathode metal, a balance must be struck between metals having the highest electron injecting efficiencies and those having the highest levels of stability. The highest electron injecting efficiencies are obtained with alkali metals, which are too unstable for convenient use, while metals having the highest stabilities show limited electron injection efficiencies and are, in fact, better suited for anode construction.
Despite improvements in the construction of organic EL devices, a persistent problem has been dark spot formation in environments in which the organic EL device is exposed to some level of moisture in the ambient atmosphere. Microscopic analysis of organic EL devices exhibiting dark spot behavior has revealed oxidation of the cathode occurring at its interface with the organic electroluminescent medium. It is believed that the oxidation of the cathode metal at its interface with the organic electroluminescent medium creates a resistive barrier to current flow in affected areas of the organic EL device. Without current flow in an area of the organic EL device, no electroluminescence can occur, and the result is seen as a dark spot when other areas of the organic EL device are emitting.
Indium has been employed as a cathode for organic EL devices. Tang U.S. Pat. No. 4,356,429 teaches to form cathodes of organic EL devices of metals such as indium, silver, tin, and aluminum. Van Slyke et al U.S. Pat. No. 4,539,507 shows a specific organic EL device construction containing an indium cathode.
Tang et al U.S. Pat. No. 4,885,211 teaches to form the cathodes of organic EL devices of a combination of metals, with at least 50 percent (atomic basis) of the cathode being accounted for by a metal having a work function of less than 4.0 eV. While both magnesium and indium are metals having a work function of less than 4.0 eV, indium has a slightly higher (+3.8 eV) work function than magnesium (+3.7 eV) and for this reason Tang et al prefers electrodes containing predominantly magnesium (since cathodes containing entirely of magnesium are unstable). In Table IV Tang et al compares the efficiency of a Mg:In (11.5%, atomic basis, In) with a pure In cathode. The indium cathode exhibits &lt;40% of the efficiency of the Mg:In cathode. In Example 12 Tang et al demonstrates declining efficiencies as a function of time with Mg:Ag cathodes. Since indium has a lower work function than silver (4.0-4.5 eV), substituting indium for silver in Table V of Tang et al would be expected to produce an inferior EL device.